To print ever smaller features of the patterns defining integrated circuits onto semiconductor wafers, the wavelength of the light to project image of a pattern onto the wafer is continuously reduced. Extreme Ultraviolet lithography (“EUVL”) is one of the lithography technologies, which employs short wavelength radiation (“light”) in the approximate range of 10 nanometers (“nm”) to 20 nm enabling to print features having a size below 100 nm. Because extreme ultraviolet (“EUV”) radiation is absorbed in almost all materials, the optics utilized in EUVL systems is reflective.
Typically, to produce EUV photons plasma is created in a source of EUV radiation containing a fuel material. The plasma is created through an electrical discharge or by a laser. EUV photons are produced by radiation of the charged atoms of the plasma. An amount of produced EUV photons depends on a conversion efficiency of the fuel material. The conversion efficiency indicates how much input energy is needed to produce a EUV photon and is defined as a ratio of output EUV power to the input power. Typically, a xenon gas having a conversion efficiency of 1% is used to produce EUV photons.
Lithium (“Li”) or tin (“Sn”) has approximately 2 to 4 times higher conversion efficiency over the xenon gas to produce EUV photons. In other words, lithium or tin may make several times more EUV light than xenon for the same input energy.
Lithium or tin, however, are reactive materials and tend to chemically attack all nearby surfaces, and in particular, the fragile EUV collector optics. The lithium or tin atoms in the plasma are evaporated as the plasma heats up. The lithium or tin atoms tend to condense on the surface of the optics making EUV optics inoperable. For example, Li or Sn buildups having a thickness of about 10 nanometers (“nm”) reduce the reflectivity of the reflective surface of the optics down to zero.
Maintaining the EUV optics at elevated temperatures to keep the evaporation rate of the Li or Sn atoms at the same level as the condensation rate does not remove contaminants from the optics, because at elevated temperatures, the Li or Sn atoms start to diffuse into the material of the optics. Diffusion of lithium or tin atoms into the optical material destroys optical coating of the EUV optics that makes EUV optics inoperable.